High harmonic generation light source with polarization selectivity and sub-100-$μ$m beam size for time- and angle-resolved photoemission spectroscopy

TL;DR

This paper tackles the need for high-energy, ultrafast TrARPES probes with small beam footprints and polarization control. It presents a gas-cell-driven HHG source driven by the second harmonic, optimized focusing with a toroidal mirror to achieve a beam size of $57 μm × 90 μm$, and polarization selectivity between $s$- and $p$-pol. The system delivers energy resolution better than $80 meV$ and demonstrates polarization-sensitive ARPES across NbSe2, MoSe2, TiSe2, and Bi2Se3, as well as TrARPES on bilayer graphene using $2 μm$ MIR pumping with a time resolution of $140 ± 10$ fs. This combination expands the capability of TrARPES to map ultrafast dynamics and light-induced phenomena in quantum materials, including small flakes and heterostructures, and paves the way for Floquet-engineered states. The use of a 10 kHz amplifier enables efficient data acquisition and robust integration of MIR pumping for advanced pump–probe experiments.

Abstract

High-quality ultrafast light sources are critical for developing advanced time- and angle-resolved photoemission spectroscopy (TrARPES). While the application of high harmonic generation (HHG) light sources in TrARPES has increased significantly over the past decade, the optimization of the HHG probe beam size and selective control of the light polarization, which are important for TrARPES measurements, have been rarely explored. In this work, we report the implementation of high-quality HHG probe source with an optimum beam size down to 57 $μ$m $\times$ 90 $μ$m and selective light polarization control, together with mid-infrared (MIR) pumping source for TrARPES measurements using a 10 kHz amplifier laser. The selective polarization control of the HHG probe source allows to enhance bands with different orbital contributions or symmetries, as demonstrated by experimental data measured on a few representative transition metal dichalcogenide materials (TMDCs) as well as topological insulator Bi$_2$Se$_3$. Furthermore, by combining the HHG probe source with MIR pumping at 2 $μ$m wavelength, TrARPES on a bilayer graphene shows a time resolution of 140 fs, allowing to distinguish two different relaxation processes in graphene. Such high-quality HHG probe source together with the MIR pumping expands the capability of TrARPES in revealing the ultrafast dynamics and light-induced emerging phenomena in quantum materials.

Figures (5)

• Figure 1: A schematic layout of TrARPES setup with HHG probe and MIR pump light sources. (a) A schematic layout of the TrARPES setup, including HHG probe, forth harmonic generation (FHG) probe and MIR pump. BS: beam splitter; FM: flip mirror; WP: waveplate; DS: delay stage. The incident angle is 80$^\circ$, 85$^\circ$ and 85$^\circ$ for gold mirror 1, toroidal mirror and gold mirror 2. (b) Engineering drawing of the HHG beamline. The inset shows two HHG setups using capillary and gas cell, respectively.
• Figure 2: Characterization of the HHG source. (a) and (b) Photos of gas cell and capillary during operation. (c, d) Photon flux of the HHG source as a function of Ar pressure generated by (c)gas cell and (d) capillary, where the input SH is 150 $\mu$J and the generated HHG source passes through a 0.2 $\mu$m thick Al film and a Au mirror (grazing incidence). (e) Photoemission spectrum of polycrystalline gold measured by HHG light generated by gas cell, which shows a clear Fermi edge. (f) Energy distribution curve (EDC) extracted from (e), which is fitted by a Fermi-Dirac distribution. The photon energy is fitted to be 21.78 eV (17.48 eV Fermi energy plus a 4.3 eV work function). The total energy resolution is 76 $\pm$ 20 meV after subtracting thermal broadening of 28 meV at the measurement temperature of 80 K and an analyzer broadening of 40 meV. (g) Electronic structure of bulk WSe$_2$ at K point, which shows sharp spectrum. (h) EDC extracted from (g), which is fitted by Gaussian function. The total energy energy resolution is 71 $\pm$ 2 meV after subtracting thermal broadening at 80 K and an analyzer broadening .
• Figure 3: Optimization of beam focusing using toroidal mirror. (a) The schematic of a toroidal mirror, which processes three degrees of freedom: R1, R2 and R3 rotation. (b--d) Photos of the laser pointer beam at focal point as function of R1--R3 rotations (the white lines are uniform scale bars for R1--R3 which represent the same length). (e) Photo of Au pattern (thickness is 100 nm) deposited on insulating SiO$_2$ which is used to scan the HHG beam size in two directions. The edge of the Au pattern is sharper than 1 $\mu$m. (f) and (g) The photoemission intensity obtained from horizontal (red) and vertical (blue) line cuts as marked in (e). The intensity contrast is fitted by convolution of a step function and a Gaussian function. (h) Vertical beam size as a function of R2 rotation.
• Figure 4: Electronic structure of various quantum materials measured by the HHG source. (a) Energy contours of NbSe$_2$. (b)--(i) Experimental electronic structure of various various TMDCs including NbSe$_2$, MoSe$_2$, TiSe$_2$, and topological insulator Bi$_2$Se$_3$ using $s-pol.$ (top rows) and $p-pol.$ (bottom rows) HHG polarizations respectively. The measurement direction is along $\Gamma$-K for (b,d,h) and $\Gamma$-M direction for (f).
• Figure 5: Ultrafast dynamics of bilayer graphene revealed by TrARPES upon 2 $\mu$m pumping. (a) Dispersion image of epitaxy bilayer graphene at K point measured by $p-pol.$ HHG photons. Inset: Brillouin zone with measurement direction (red line). (b)--(f) TrARPES dispersion images of bilayer graphene at different delay times. The pump photon energy is 540 meV and the pump fluence is 1 mJ cm$^{-2}$ with $p-pol.$ (g) Evolution of momentum-integrated intensity with energy and delay times. (h) TrARPES intensity as a function of delay times at 0.5 eV. The red curve is the fitted Gaussian function for extracting the time resolution. (i) EDCs at different delay times from (g) and corresponding Fermi-Dirac fitting. (j) Extracted electronic temperature as a function of delay time.